This is a Solidity smart contract used for locking liquidity from a Uniswap V3 liquidity pool.

Background

Uniswap V3 pools differ from V2 pools in a number of ways. One difference that is significant to the implementation of a lock contract such as this is the mechanism of proof of ownership of liquidity.

When depositing liquidity in Uniswap V2, you are issued some number of ERC-20 tokens called liquidity tokens. To retrieve the underlying liquidity, you must "burn" some or all of these tokens, exchanging them for their portion of the liquidity pool. These tokens are "fungible" meaning the individual tokens are not unique. Any one liquidity token from a particular pool is equivalent to any other token from that same pool.

Uniswap V3 works a little differently. Because of the parameters involved when depositing liquidity in a V3 pool, my liquidity is not directly equivalent to your liquidity in the same pool, as is the case in V2. For example, I might choose to incur higher risk than you in exchange for higher fees. Because each liquidity position in a pool is unique, V3 pools issue an ERC-721 Non-Fungible Token, a.k.a an "NFT" instead of ERC-20 tokens. This unique token corresponds to my unique position within the pool. If I want to decrease my liquidity position in the pool, I must identify myself as the holder of the NFT that corresponds with the position. If I want to transfer control of my position to another person or wallet, I can simply transfer my NFT to that account.

Implementation

This last point is how the Liquidity Lock contract works. To lock your liquidity position, you must transfer your Uniswap NFT to the lock contract. In return, the contract mints a new NFT that it transfers to you. This new NFT represents your ownership of the locked position. From there, the contract acts as a simple wrapper around Uniswap's NonfungiblePositionManager contract. You can collect fees at any time, just like you could if you still had direct ownership of the position, but you cannot decrease your position until the parameters of the lock are satisfied.

When locking a position, you specify two parameters: A "start unlocking" timestamp, and a "finish unlocking" timestamp. The first timestamp indicates when your locked liquidity starts to become available for withdrawal. Until this date has elapsed, you cannot decrease your position at all. After this date has passed, your liquidity begins to unlock gradually (and linearly) until the "finish unlocking" date has passed. At this point, your entire liquidity position is unlocked and you are free to decrease your position all the way to zero. You can even request that your original Uniswap NFT be returned to your account and for your lock NFT to be burned.

Usage

To lock your Uniswap V3 liquidity position, start by transferring your Uniswap NFT to the lock contract. Though there are a few APIs you can use on the ERC-721 contract to transfer an NFT, you MUST use the following API:

safeTransferFrom(address from, address to, uint256 tokenId, bytes data)

This requirement is in place for two reasons:

1. The safeTransfer APIs cause the ERC-721 contract to call the onERC721Received function of the lock contract, but the transfer functions do not. Since this "on received" function is where the lock NFTs are minted, not calling it would result in your liquidity token becoming permanently and irretrievably locked.
2. You must use this specific variant of safeTransferFrom because you are required to include the data field. Specifically, this field should contain the "start" and "finish" timestamps. To encode these timestamps into data, you can use the web3.eth.abi.encodeParameters function of web3.js, for example:

const web3 = new Web3;
const encodedTimestamps = web3.eth.abi.encodeParameters(
    ['uint256', 'uint256'],
    [1672531200, 1688169600] // 1/1/2023 -> 7/1/2023
);

After successfully transferring your Uniswap token to the lock contract, you will now own a new NFT minted by the lock contract. To get your token ID, call this function, passing the token ID of the Uniswap token you just transferred:

getLockTokenId(uint256 uniswapTokenId)

This new token ID is the one you will use when performing actions on the contract such as collecting fees or decreasing liquidity.

Contract addresses

Network	Address
Mainnet	Not yet deployed
Rinkeby	0x2236FF5073C5e8A5ef5a9C5d8a29C1e808DE4Bb4

License

The code is available under the MIT license. See LICENSE.md for more information.